1. Field of the Invention
This invention relates generally to an apparatus and method for generating output signals of different phases. More particularly, this invention relates apparatus and method for generating quadrature signals in reference to a virtual ground to achieve total symmetry for these quadrature signals so generated.
2. Description of the Prior Art
As more portable personal communication systems are made available as a result of recent progress in the semiconductor technology and packaging engineering, one difficulty often encountered is to establish a virtual ground potential for such portable communication devices. As these communication devices often require broad bandwidth, the phase splitters which may be operated in the frequency ranges of more than few GHz are commonly utilized in such systems. The difficulty for the phase shifters to define a virtual ground position especially for application in portable communication systems presents a special problem for the design of such system when the signal process has to be performed without precise and stable reference voltage for reference as the ground voltage.
FIG. 1 shows a conventional phase splitter 10. The phase splitter 10 receives an input voltage V.sub.in through a field effect transistor (FET) T.sub.in which is connected in a source follower configuration to two branches of circuits wherein each of these branches forms a mutually inverted configurations of like valued elements including a resistor and a capacitor. A first branch is shown on the upper portion of the shifter 10 wherein the source of the FET T.sub.in is first connected to a capacitor C.sub.1 and then to a resistor R1 before it is connected to the ground. An output is derived from the output end of the capacitor C.sub.1 for connecting to the gate of an output FET T.sub.o. A second branch is shown in the bottom portion of the shifter 10 wherein the input line as connected from the source of the input FET T.sub.in is first connected in series to a resistor R.sub.2 and then to a capacitor C.sub.2 before it is connected to the ground. An output is derived from the output end of the resistor R.sub.2 for connecting to the gate of an output FET T.sub.o '. In arranging the phase shifter 10 by making R.sub.1 equals to R.sub.2 and C.sub.1 to C.sub.2, the output voltage as obtained from the source of the output FET T.sub.o ' has a ninety degree phase difference with that of the output voltage of the FET T.sub.0.
The conventional phase shifting circuit 10 presents two problems. First, as the output end of the capacitor C.sub.1, i.e., point 12 is now brought to a ground potential via the resistor R.sub.1 when there is no current flows through that branch of circuit. The direct current (DC) reference voltage for the in-phase output from the FET T.sub.o and that for the output from the out-of-phase output from the FET T.sub.o ' cannot be consistently maintained at the same level. Secondly, for operation at high frequency, the ground potential for this phase shifter 10 is difficulty to measure and define, which makes it very inconvenient to utilize this shifter in a portable communication system. The situation is even worse when the shifting circuits are fabricated as integrated circuits (ICs) on an IC chip.
Altes discloses in U.S. Pat. No. 4,857,777 entitled "Monolithic Microwave Phase Shifting Network" (Issued on Aug. 15, 1989) a phase shifting network with multiple outputs. A constant phase difference is maintained between these outputs. The configuration of this network is a four-branch all-pass network. It has four similar all-pass networks. An in-phase and an out of phase input signals are applied to this network to generate four signals in phase quadrature of zero, ninety, one-hundred and eighty, and two hundred and seventy degrees. The phase shifter is intended for use with micro and millimeter wavelengths and may be realized from resistive and capacitive elements fabricated on a common monolithic substrate.
The phase shifting network as disclosed by Altes is able to eliminate the need of a reference ground potential for the RC networks. However, the quadrature shifting is accomplished via two RC branches that use resistors and capacitors of different values of resistances and capacitances in two branches. The circuits would be susceptible to fabrication variations due to these difference between the branches. The accuracy in fabricating the phase shifting circuit with controlled operation characteristics is therefore limited. Due to these limitations, it is difficult to design and manufacture the shifter for generating phase shift outputs with precisely controlled accuracy. Furthermore, application of this type of phase shifters to portable devices would be inconvenient and probably more expensive if additional circuit elements have to added for establishing and determining a stable and measurable virtual ground voltage.
Therefore, there is still a demand in the art of design and manufacture of phase shifting devices, particularly for portable devices for application to higher bandwidth ranges, an improved phase shifting apparatus and method to overcome this difficulty.